The present invention relates to a communication control unit which allows the effective utilization of a single communication line by dynamically selecting various types of communication protocols, and more particularly to a protocol selector and a protocol selection method in a communication control unit which allows the selection of a communication protocol by negotiation with a communicating communication control unit.
In the past, the communication between terminal devices or between a host and a terminal device has been done by utilizing a closed area network or an exclusive line network which operates with a switching system connecting those terminal devices or host. Accordingly, the communication protocol used for each equipment is fixedly determined for each equipment. As a result, communication between equipments having different communication protocols is in fact not attained or possible in such systems.
A prior art for solving the above problem is disclosed in JP-A-61-263352 and JP-A-61-127251. In the system disclosed in JP-A-61-263352, where a plurality of types of different terminal devices are connected on a subscriber line, means for indicating the usage status of the respective terminal devices to a calling terminal device when the terminal devices receive a call is provided on the subscriber line so that the calling terminal device may execute a communicatable call set request. Namely, means for indicating a communicatable protocol from the receiving terminal device is used to select the protocol in the calling terminal device.
In the art disclosed in JP-A-61-127251, where a terminal device which can process a plurality of protocols is connected to a subscriber line, a program which corresponds to the protocol designated by a calling terminal device is loaded when the call is received so that a plurality of protocol processes can be executed on a single subscriber line. Namely, a communicatable protocol is recognized by the called terminal device which will then select and execute the protocol.
However, in the system disclosed in JP-A-61-263352, only means for allowing the calling terminal device to select the communicatable communication protocol unit of the called terminal device is available when the calling terminal device calls, and no consideration is given to the dynamic selection of the communication protocol of the calling terminal device by the protocol of the communication control unit of the destination terminal device when terminal device is called or calls. Accordingly, the type of destination terminal device which is communicatable with a calling terminal device is limited.
In the prior art communication control unit, the selection and initialization information of the protocols for the respective layers which process a communication frame is one which has been preset by a user prior to the communication. Such information stored in an auxiliary memory is fetched at the time of start of the communication program to effect an initialization. Accordingly, the generated port cannot operate in other than the initially set protocol.
Where the above system is applied to an ISDN in which the utilization format of the communication line is negotiable, the communication protocol is fixedly determined for each B channel which is the communication port, and it is not possible to select an optimum line utilization format in accordance with a request of an application program or effect communication in response to a communication request if a vacant generated B channel which matches a protocol of the destination terminal device is not available.
Further, in the system disclosed in JP-A-61-263352, only the means for dynamically selecting the protocol designated by the calling terminal device, in the called terminal device when it receives a call is provided, and no consideration is given to simultaneous operation of a plurality of protocols in one communication layer. Accordingly, during communication with a called terminal device, the communication in another protocol cannot be attained.
It is an object of the present invention to solve the problems encountered in the prior art units and provide a protocol selector and a protocol selection method in a communication control unit which can dynamically determine a protocol of a communication port, set a coupling point of an upper layer and a lower layer so that simultaneous communication with various communication control units is attained.